It is a commom misconception that the fan and shroud do nothing at hiway speeds. This is untrue. At highway speed airflow tends to stall and get turbulent in the grill area, the fan, which is more efficient because it has a shroud, provides a low pressure area to pull the air through the radiator, instead of allowing it to spill out under the car and over the radiator yoke. Ask anyone who has ever towed on a grade at hiway speed and see if their temp started creeping up, then you hear the fan clutch engage and watch the temp come down. This would not happen if the fan did nothing at hiway speed.